Recent developments in the growth of a new class of materials made by several laboratories world-wide promise to expand the ability to engineer materials to obtain a broad range of optical and electronic properties optimized for numerous applications. This class of material combinations have a single feature in common: Layers of differing material compositions are bonded to each other via the highly flexible van der Waals force. Chief among such van der Waals solids are organic molecular semiconductors which have recently been demonstrated to grow epitaxially on potassium-halide crystals, sodium chloride, semiconductors, and metal substrates. Furthermore, combinations of two different van der Waals crystals with vastly different lattice constants have been observed by the inventors to grow in ordered, multilayer stacks with layer thicknesses of only a few .ANG.ngstroms.
These so-called quasi-epitaxial structures have enormous promise for use in a wide range of optical and electronic devices due to their non-linear optical and electronic properties. Indeed, the recent demonstration of quasi-epitaxy is particularly exciting, since it suggests that van der Waals bonds are sufficiently flexible to allow lattice-mismatched materials to form crystalline structures without inducing a high density of defects. This is in contrast to inorganic heterojunction systems consisting of mismatched combinations of materials, such as InP and InGaAsP.
Thus, new opportunities for engineering heterostructures consisting of a wide range of materials chosen without regard to the lattice-matching condition presents exciting possibilities in the field of optoelectronics. It should be noted that observation of quasi-epitaxy has not been confined only to materials combinations involving organic molecular crystals. In recent work, highly mismatched layers of dichalcogenide films have been grown in a quasi-epitaxy structure onto metal, semiconductor, and mica substrates as well.